(1) Field of the Invention
The present invention relates to an optical switch, and an apparatus and method for controlling the optical switch. Particularly, the present invention relates to a technique suitable for use at the case where an optical path is deflected by changing an angle of a reflecting surface to direct a reflected beam to any one of a plurality of output ports.
(2) Description of Related Art
Heretofore, switching of channels in an optical transmission system is performed by means of an electric switch after an optical signal is converted into an electric signal. However, use of a switch (optical switch) for switching an optical signal as it is without converting the optical signal into an electric signal can improve the channel switching speed and the efficiency.
FIGS. 13, 14 and 15 show a structure of an optical switch. FIG. 13 is a schematic perspective view of an optical switch, FIG. 14 is a schematic side view of the same, and FIG. 15 is a schematic plan view of the same.
As shown in FIGS. 13 through 15, the optical switch 10 comprises, for example, a wavelength spatially dispersive optical element (wavelength spatially dispersing means) 1 for dispersing a WDM (Wavelength Division Multiplexed) spatially beam according to wavelengths, an input/output optical system (an input optical system and an output optical system) 2 having an input port (input fiber) 21 and output ports (output fibers) 22 arranged in an array, a plurality of collimator lenses [collimator lens array (collimating means)] 23, etc., a focusing lens [focusing optical system (focusing means)] 3, and a movable reflector 70 having MEMS mirrors (optical deflecting means) 4 corresponding to respective plural disperse wavelengths.
As the above wavelength spatially dispersive optical element 1, a diffraction grating of the transmission type is used, for example. The wavelength spatially dispersive optical element 1 is inputted thereto a WDM beam from the input port 21, disperses wavelength components contained in the WDM beam to different directions according to the wavelengths, and outputs the wavelength components.
The above movable reflector 70 is provided with a plurality of micro mirrors (MEMS mirrors) 4 as an optical deflecting means arranged in an array in the direction of wavelength dispersion by the element (diffraction grating) 1. Each of the MEMS mirrors 4 fulfills a function as a wavelength selecting switch, which reflects a beam irradiated on itself, which corresponds to its own position, among beams dispersed by the above element 1, and directs the beam to any one of the plural output ports 22 in the input/output optical system 2.
Selection of the output port 22 can be done by changing the angel of the reflecting surface of the MEMS mirror 4. By independently controlling the angle of the reflecting surface of each of the MEMS mirrors 4, different switching can be performed for each of a plurality of wavelengths, separately.
For example, as shown in FIGS. 14 and 15, by changing the angle of the reflecting surface of one MEMS mirror 4 so as to direct the reflected beam to a different output port 23 (for example, changing the angle of the reflecting surface along a direction in which ports 21 and 22 are arranged), it is possible to distribute a predetermined wavelength contained in the WDM beam inputted from the input port 21 to any one of the output ports 22 (for example, refer to Patent Document 1 below).
By finely changing the angel of each of the MEMS mirrors 4, not dynamically moving the angle as an output port 22 is selected, it is possible to attenuate the optical intensity inputted to the output port 22 (that is, to realize the optical attenuator function).
Each of the MEMS mirrors 4 is operable about two axes, that is, the mirror angle thereof can be changed in both a direction along wavelength dispersion direction (horizontal direction) as shown in FIG. 14, and a direction (vertical direction) perpendicular to the wavelength dispersion direction as shown in FIG. 15.
Patent Document 1: Published Japanese Translations of PCT International Publication for Patent Application No. 2003-515187
When the output port 22 is switched, the mirror angle of the MEMS mirror 4 is changed in a direction along the arrangement of the output ports 22 if a port to be switched to is an adjacent port 22. When it is necessary to switch to a port not adjacent, it is necessary to avoid a leakage of the beam to an adjacent port 22.
As schematically shown in FIG. 16, the mirror angle of the MEMS mirror 4 is first changed in the horizontal direction (a direction along the wavelength dispersion direction) to move the beam position to a position (a position at which the beam does not leak to the output port 22: this being called an optical isolation region 11) away, in the horizontal direction, from the position of an erstwhile output port (22a) (refer to an arrow A). After that, the mirror angle is changed in the vertical direction (a direction perpendicular to the wavelength dispersion direction) to move the beam position to a position corresponding to the center position of a target output port 22 (22c) (refer to an arrow B). Next, the mirror angle is again changed in the horizontal direction (but in the direction opposite to the first) to move the beam to the target output port 22 (22c), and the beam is irradiated thereon (refer to an arrow C).
According to the port switching as above, it becomes possible to decrease the distance (pitch) between the output ports 22 (to about 1.5 mm).
However, such switching operation causes the transmission band characteristic to have shapes shown in FIG. 17 when the output port is switched (when the beam is attenuated). FIG. 17 shows a relationship between the beam spot radius and the band characteristic containing an effect of diffraction, expressing a plurality of different characteristics obtained at respective angles (0°, 1.2°, 1.5° and 2.0°) of the MEMS mirror 4 when the angle of the MEMS mirror 4 is changed. In FIG. 17, the vertical axis represents the transmission intensity (dB), whereas the horizontal axis represents the wavelength band, with the width (length in the dispersion direction) of the MEMS mirror 4 being 1 (±0.5), that is, a normalized wavelength band. FIG. 17 indicates that, as the angle of the MEMS mirror 4 increases, convex portions of the transmission band characteristic rising from the flat portion in the middle generates, that is, protuberances in the vicinity of the out-bands (the side lobes) of the transmission band generates.
FIG. 18 shows the principle of this.
It is assumed that each beam having been dispersed by the wavelength spatially dispersive optical element 1 is the center wavelength, each MEMS mirror 4 is so set that each beam having the center wavelength is irradiated onto the center position of the MEMS mirror 4. If each wavelength contained in the WDM beam is not practically shifted from the center wavelength, each beam is irradiated onto the center position of the MEMS mirror 4, as denoted by a reference character 5c in a balloon 100 in FIG. 18.
When the center wavelength of each dispersed beam is shifted (when the beam contains a shifted component), the beam is irradiated onto a position denoted by a reference character 5b or 5d in the balloon 100 in FIG. 18. When the beam is further shifted, the beam is irradiated onto the edge's side of the MEMS mirror 4 (the edge's side of the MEMS mirror 4 with respect to the wavelength dispersion direction) as denoted by a reference character 5a or 5e in the balloon 100 in FIG. 18.
Here, attention should be given to that diffraction generates in the reflected beam 6 (6a, 6e) when the incident beam is irradiated onto a portion in the vicinity of the edge of the MEMS mirror 4 as shown in the balloon 100 in FIG. 18 because a part of the incident beam 5 (5a, 5e) is cut, thus the spot radius increases as compared with the reflected beam 6 (6b through 6d) at the time that the beam is irradiated onto a portion in the vicinity of the center of the MEMS mirror 4.
When the spot radius increases as above, the side lobe of the reflected beam 6 is lifted (refer to a curve 9) as shown in a balloon 200 in FIG. 18. In the side lobe portion, the beam power is larger than the power (refer to a curve 8) of the reflected beam 6 reflected from a portion in the vicinity of the center of the MEMS mirror 4. Namely, when the reflecting surface of the MEMS mirror is inclined toward the wavelength dispersion direction, the incident beam is further cut at the edge. However, this inclination of the reflecting surface causes the power of the side lobe portion of the reflected beam 6 to be cut by the aperture (area) 7 of the collimator lens 23 (that is, an area included in a portion denoted by a reference character 7 is the transmission intensity of the wavelength), thus the output beam intensity from a portion in the vicinity of the edge of the MEMS mirror 4 is further increases.
Namely, since the nearer the edge of the MEMS mirror 4, the more the cut quantity of the reflected beam 6 is, the effect of diffraction increases at the time of switching of port in the wavelength dispersion direction, as described above. If there is no effect of the diffraction, the transmission band characteristic would be trapezoidal because there is only a change in beam power caused by that the reflected beam 6 is cut. However, the effect of diffraction adds a trapezoidal transmission band characteristic, as shown in FIGS. 17 and 18.
Meanwhile, why the peaks of the reflected beams 6 at difference wavelengths are at the same position in space (spatial separation quantity=0) is that the reflected beams 6 are focused by the focusing optical system 3 and the beams are made parallel by changing the angles by the wavelength dispersive optical element 1, as shown in balloons 300, 400, 500 and 600 in FIG. 18. The width of the aperture 7 shown in the balloon 200 in FIG. 18 shows an aperture to the output optical system 2 composed of mainly the collimator lens 23. The width of the aperture 7 increases, correspondingly to the area of the collimator lens 23.
When a convex transmission band characteristic above the flat portion in the middle (that is, a protuberance in the vicinity of the out-band (the side lobe) of the transmission band generates, the convex portion is also amplified in optical amplification by an optical amplifier when this is used in an optical system. This causes degradation of the S/N ratio. This becomes noticeable in multi-stage connections, which limits the number of the multi-stage connections, preventing the system structure from having high freedom.